Diagnosis and treatment of many medical illnesses require monitoring and recording biopotentials of a patient. Biopotentials are voltages generated between points in living tissue. An example of a biopotential is the electrical activity of the heart that is typically recorded in the form of an electrocardiogram (ECG). The electrical activity of the heart is associated with both ion movement and ion concentration, as opposed to electron movement. Skin-mounted monitoring electrodes are used to transduce the ion movement into electron movement to be displayed as voltage or a time-dependent function of voltage called a signal. The signal is a composite of several potentials, in addition to those generated by the heart. These potentials include the half-cell potential of the electrode and the potential created by the ion concentration gradient developed across the skin, referred to as the skin potential.
It is well known in the medical field that there are a number of problems associated with monitoring biopotentials through the use of skin-mounted monitoring electrodes. One such problem is related to an unwanted signal that is created by skin motion under the electrodes. Skin has a complex electrical impedance that is inhomogeneous over the extent of the skin surface. The skin is composed of two principal layers; namely, the epidermis and the dermis. The epidermis includes the stratum corneum or horny layer as the outermost layer of skin, the stratum granulosum which is adjacent the stratum corneum, and the stratum germinativum or innermost layer of the epidermis. The stratum corneum is composed of keratin containing dead cells that have lost their nuclei. These cells are approximately 0.5 to 0.8 micrometers (.mu.m) thick and about 3 .mu.m in diameter. The stratum corneum varies in thickness from approximately 15 cell layers thick on the abdomen, 45 layers thick on the arms, to upwards of several hundred layers thick on the palms and soles. An additional stratum lucidum layer is found in the skin of the palm (hand) and sole (foot) and lies directly beneath the stratum corneum. The dermis is composed of loose connective tissue which contains numerous capillaries, lymphatics, nerve endings, hair follicles, sebaceous glands, sweat glands, and smooth muscle fibers.
The major contributor to skin impedance is associated with the stratum corneum. When the skin is deformed by either compression or extension, a variation in both the skin impedance and the potential between the inside and outside layers of the skin occurs. This variation in skin potential is referred to as a motion artifact or wandering baseline and cannot be easily separated from the biopotential signal of interest. If the motion artifact is severe enough it can mask or hinder the correct diagnosis of the patient's condition. Motion artifact problems are commonly associated with patient's undergoing stress testing, ambulation, or emergency transport whereby motion of the patient causes disturbance of the skin underneath the electrodes.
It is commonly known among medical practitioners that light abrasion or penetration of a patient's skin reduces skin impedance and thereby reduces the effect of skin motion. Methods of skin abrasion have included the use of sandpaper, SCOTCH-BRITE.TM. abrasive pads, blood lancets, gels containing abrasives, brisk rubs with gauze pads or brushes, and even dental burrs. Methods of skin preparation vary among practitioners; however, the effectiveness of each procedure depends upon the skill of the practitioner and time available. If the skin preparation that was performed is inadequate, the electrode has to be removed, the skin site has to be reprepped, and a new electrode has to be reattached.
Various electrode systems have automated skin electrode application and skin preparation, including an abrading disc and motorized applicator gun as disclosed in Modes et al., U.S. Pat. No. 4,311,152. However, the electrode and applicator gun are complex, relatively expensive and not without some skin trauma in use. Other art has suggested piercing the skin as a method of skin preparation, for example, Lahr, U.S. Pat. No. 3,774,592, and Lewes et al., U.S. Pat. No. 3,505,993. Lahr employs a specialized electrode that includes an absorbent pad saturated with a conductive material. The electrode is adhesively affixed to the skin. The bristles of a separate brush are applied to the back of the electrode and forced through the saturated absorbent pad and into the skin. This method generally results in reliable contact with the skin; however, the method requires a practitioner to perform a number of steps and does not limit the penetration depth of the bristles into the skin. Lewes et al. disclose an electrode that also pierces the skin using rigid metal projections which penetrate into the epidermis. A disadvantage of the electrode disclosed in Lewes et al. is that the projections remain in the skin during the measurement, which could result in undue laceration of the skin should the electrode move with respect to the skin and also contribute spuriously to the electrode half-cell potential.